Silicon accelerometer

• US 4,553,436 A
• Filed: 11/09/1982
• Issued: 11/19/1985
• Est. Priority Date: 11/09/1982
• Status: Expired due to Fees

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Hansson, Jan I.
• Primary Examiner(s): Gill, James J.
• Application Number:
  US06/440,462
• Time in Patent Office: 1,106 Days
• Field of Search: 73/516 R, 73/517 R, 338/43, 338/46, 338/47
• US Class Current: 73/514.33
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/123 by piezo-resistive elements, e.g. semiconductor strain gauges

Abstract

A silicon accelerometer employing the piezoresistive effect of single crystal silicon to measure the flexure of semiconductor beams supporting a semiconductor mass. In one embodiment a rectangular semiconductor center mass is supported at each corner by a semiconductor beam parallel to one side of the center mass and perpendicular to the adjacent beams, each of the beams having an implanted resistor at the stationary end thereof. The crystal planes and relative orientations of the resistors are selected so that two resistors always increase, and two always decrease their resistance by the same amount as the center mass is displaced, which allows them to be connected in a Wheatstone bridge having a symmetric differential output.

Claims

1. An acceleration sensor comprising:

a substantially planar semiconductor support frame;

four flexible, elongate semiconductor support members each rigidly attached at a first end to said support frame, and disposed substantially perpendicular to each adjacent support member;

a semiconductor deflection mass positioned internally of and movable relative to said support frame, in a direction perpendicular to the plane of said support frame, and attached to a second end of said support members; and

piezoresistive means for sensing stress in said support members, said piezoresistive means oriented with respect to a crystalline plane and capable of providing a differential output as a function of the magnitude and direction of bending of said support members.

2. A sensor as set forth in claim 1 wherein said differential output comprises a Wheatstone bridge.

3. A method for fabricating an acceleration sensor comprising the steps of:

providing a semiconductor substrate of one conductivity type;

doping impurities of the opposite conductivity type into an upper surface of said substrate to form a buried layer therein;

forming an epitaxial layer of the first conductivity type on the upper surface of said substrate over said buried layer;

forming a first insulating layer on the surface of said epitaxial layer;

forming a pattern of openings in said first insulating layer extending to the surface of said epitaxial layer, wherein the pattern of openings defines locations for piezoelectric resistive elements;

doping impurities of the opposite conductivity type into the surface of said epitaxial layer within the pattern of openings to form doped resistive element regions;

forming a second insulating layer on the surfaces of said first insulating layer, said epitaxial layer and said doped resistive element regions;

forming a pattern of openings in said second insulating layer;

forming a pattern of conducting elements over said pattern of openings in said second insulating layer wherein the conducting elements make contact with the resistive element regions;

forming a third insulating layer on a lower surface of said substrate and on the exposed surfaces of said second insulating layer and said conducting elements;

forming a pattern of openings in said third insulating layer on the lower surface of the substrate to define locations for support members and for a deflection mass;

etching said substrate on the lower side to form a pattern of channels therein extending to said buried layer;

forming a pattern of openings in said third insulating layer adjacent said epitaxial layer aligned with said channels to define the support members and the deflection mass;

etching said third insulating layer, said epitaxial layer and said buried layer to separate the deflection mass from the remainder of the substrate, and to separate the support members from the remainder of the substrate and from the deflection mass; and

removing said third and fourth insulating layers.

4. The method of claim 3 wherein said first etching step comprises an orientation dependent etch.

5. The method of claim 4, further including the steps of:

attaching said sensor to a header; and

affixing a cover to the surface of said sensor opposite said header and said cover by means of a nonconducting adhesive compound including a plurality of nonconducting spacer elements therein of a predetermined size.

6. The method of claim 4 wherein said step of forming a third insulating layer comprises forming a layer of compressive nitride.